1. Field of the Invention
The invention relates to a method and structure for semiconductor devices, and more particularly to a method and structure for optical semiconductor devices.
2. Description of the Prior Art
Semiconductor devices are employed in a wide variety of electrical applications, for example, in central processors, memory devices, microwave devices, and light emitting devices.
Considering the amount of heat generated by a semiconductor device, this can heat up the device and, as a result, lead to a reduction in device lifetime and reliability. In addition, for optical semiconductor devices, such as light emitting diode (LED), edge emitting laser, or vertical cavity surface emitting laser, the light emitting efficiency can also be greatly reduced. In case of AlGaInP-based LED, the light is emitted via the direct bandgap recombination of electrons at  with holes. At increased device temperatures, a substantial amount of electrons can transfer to  band and then indirectly be combined with holes along with the generation of heat and as a result, the device internal quantum efficiency becomes lower. Consequently, the light emitting efficiency is reduced as the temperature becomes higher.
Many conventional light-emitting diodes (LEDs) are grown on a substrate having an energy gap smaller than that of the light emitting layer of the LED. The substrate can absorb some of the light generated within the active region, thereby reducing the efficiency of the device. An example of a prior art AlGaInP LED of the double heterojunction type is shown in FIG. 1. A layer 112 of n-type (AlxGa1xe2x88x92x)0.5In0.5P, a light generation layer 114 of (AlxGa1xe2x88x92x)0.5In0.5P and a layer 116 of p-type (AlxGa1xe2x88x92x)0.5In0.5P are epitaxially grown on an n-type substrate 110 where xe2x80x9cxxe2x80x9d represents the chemical composition of Al. A double heterojunction structure as a region of light emitting are formed between layers 112-116. An optically transparent current spreading layer 118 of p-type AlxGa1xe2x88x92x As or GaP is grown on the layer 116. The optically transparent current spreading layer 118 enhances lateral electrical conductivity a of p-type region and further improves current spreading on the double heterojunction structure. The amount of xe2x80x9cxxe2x80x9d in the light generation layer 114 determines the wavelength of light emitting. The bandgaps of the epitaxial layer 112, 116, and the optically transparent current spreading layer 118 are chosen so that the emitted lights will not be absorbed by these layers. However, the GaAs substrate 110 does absorb visible light. In this case, the efficiency of the light emitting device is greatly reduced because a substantial amount of downward emitted light is absorbed by the substrate.
Currently, there are several techniques for resolving the problem of light absorption by the substrate. A first technique is to grow the light-emitting devices on a non-absorbing substrate. However, the choice of accepted substrate can be quite limited, since the lattice constant of the substrate has to be very close to that of the epilayer, if not the same. A second technique is to grow a distributed Bragg reflector (DBR) between the LED epitaxial layers and the substrate. However, the improvement is limited because the distributed Bragg reflector only reflects light that is of near normal incidence. A significant amount of light that deviates from a normal incidence can pass through the DBR toward the substrate, where it is absorbed.
The third technique is to grow the LED epitaxial layers on a temporary substrate, which can be an absorbing one and is removed after epitaxy. In this case, a thick, optically transparent and electrically conductive transparent xe2x80x9csubstratexe2x80x9d is grown on the temporary substrate as the permanent one and is followed by a light emitting structure. The temporary substrate is then removed by a method of polishing, etching, or wafer lift-off. The resultant wafers are thin and fragile. Therefore, a rather thick epitaxially-grown permanent substrate is required. However, a xe2x80x9cthickxe2x80x9d transparent substrate requires a long growth time, and thus, the manufacturing throughput of such LEDs is low and the cost remains high.
Another concern of the LED performance is the current distribution. For example as depicted in FIG. 2, an n-type ohmic contact 130 that contains a composition of Au/Ge is made at the back of an n-type substrate 132; a light emitting layer 134 is on the n-type substrate 132, which can be a structure of single or double heterojunction, or a structure of multiple-quantum well; a p-type transparent current spreading layer 136 is grown on the light emitting layer 134. A bonding pad 138 of p-type ohmic contact generally contains a composition of Au/Be or Au/Zn. As current travels from the bonding pad 138 to the p-type transparent current spreading layer 136, part of the current travels laterally within the p-type transparent current spreading layer 136 and then downward through the light emitting layer 134 and emits light. On the other hand, part of the current travels directly downward from the bonding pad 138 through the p-type transparent current spreading layer 136 and the light emitting layer 134 and emits light. However, in such case the upward emitted light is blocked by the bonding pad 138 and cannot escape from the device. Therefore, this part of the current is regarded as ineffective current and should be minimized. To prevent this, a current blocking structure right below the bonding pad such that the current cannot pass directly downward to the light emitting layer 134 is necessary. One commonly used approach, based on an n-type substrate structure, is shown in FIG. 3. A current blocking structure 140 of n-type layer, whose conductivity is different from that of the transparent current spreading layer 136, is utilized to achieve the effect of current blocking. There are two methods of fabricating such a current blocking structure 140. Firstly, a two-step epitaxy method is used. It is to grow in sequence the light emitting layer 134 and then the current blocking layer and on the substrate 132. The epi-wafer is then removed from the growth chamber and the current blocking layer is etched to form the current blocking 140 structure of n-type layer. The processed wafer is put back to the growth chamber for the growth of the transparent current spreading layer 136. However, in this case, the epitaxy chamber is susceptible to pollution that can adversely affect the properties of epitaxial layers. A second method is to utilize localized selective diffusion. Though this process is simple and low cost, it is difficult to control.
It is an object of the present invention to provide a method for forming a metal substrate to replace the conventional semiconductor substrate. The reliability and lifetime of a semiconductor device can be enhanced by the high thermal and electrical conductivity of the metal substrate. In addition, the metal substrate can also enhance the efficiency of light output for an optical semiconductor device.
Another object of the present invention is to provide a method for forming a mirror-like or a rough surface between the metal substrate and the semiconductor layers for a light-emitting device. This surface may be formed by a metal/semiconductor interface or by utilizing differences of refractive index. This surface can redirect the downward emitted lights to and escape from the surface of the device so as to enhance the efficiency of emitting light.
It is yet an object of the present invention to provide a technique of a metal substrate such that current blocking layer below the light emitting layer is provided to block current and to enhance the efficiency of light emitting.
It is yet another object of the present invention to provide a method for forming a metal substrate as a temporary substrate whereby thin layers of semiconductor may be removed for other applications.
In the present invention, a method for forming a semiconductor device with a metal substrate is disclosed. The method includes providing at least using one semiconductor substrate as the temporary substrate; forming at least one semiconductor layer on the semiconductor substrate; forming a metal substrate on the semiconductor substrate and then removing the semiconductor substrate. The metal substrate has advantages of high thermal and electrical conductivity that can improve the reliability and lifetime of the semiconductor device.